The invention relates to an apparatus for the generation of pressure pulses in a drilling mud composition which flows downward in a drill string.
According to a known apparatus of this type (U.S. Pat. No. 3,958,217), the exterior part of the valve assembly forms a valve seat, which limits the upward motion of the primary valve in the final operational position acting as a stp, and which, with engagement of the primary valve, blocks the central flow passage for the drilling mud composition. The valve seat acting as a stop determines the final position of the primary valve. The sum of the downward hydraulic forces which are exerted on the primary valve in its starting position, when the pilot valve for the drilling mud composition is open, exceeds the sum of the upward hydraulic forces, with the result that the primary valve is immobilized in its starting position when the pilot valve is open. When the pilot valve is closed, the pressure differential is reversed, resulting in the upward motion of the primary valve until it reaches its final position as determined by the valve seat.